Caution for Overanxious Gardeners: Planting and Protecting Warm Season Vegetables

Saturday, May 5, 2007

This past weekend at our Rutgers Ag Field Day in New Brunswick, NJ, I
received many questions regarding the best time to plant vegetables. As the
weather turns warmer and the days grow longer, gardeners are anxious to get
their transplants into the garden soil. In our zeal to get started, it’s easy to
forget that we are still in early May and that the danger of frost is still
quite high.

Frost Free Dates for Warm Season Vegetables

For central NJ, it is wise to wait until mid-May to plant warm season
vegetables in your garden. Plant timing for warm season vegetables should be
delayed until the third week in May for northern NJ, and can be started the
first week in May for southern NJ. These frost –free dates can provide
guidelines but we must be aware that cooler temperatures and frosts can
sometimes occur after these dates and damage sensitive plants.

Warm Season Vegetables that Need Protection

Many warm season vegetables originated in warmer climates such as South
America. Plant origin often dictates the range of temperatures tolerated by
plants within the same family. For instance, warm season solenaceous plants such
as tomatoes, peppers, eggplant and potatoes or cucurbits such as cucumbers,
melons, squash, and gourds grow best in the warmer temperatures of late spring
to summer.

There is some variation in plant sensitivity to temperature within families.
For instance, tomatoes can tolerate slightly cooler temperatures than peppers or
eggplant. The group referred to as tender vegetables can be injured by light
frosts and will grow very poorly in cool temperatures. The tender vegetables
include snap beans, tomato, sweet corn and sweet potato. There is also a group
of very tender vegetables that are the most sensitive to cooler temperatures and
require the warmer soil temperatures of late May to early June for proper
growth. The very tender vegetable group includes eggplant, pepper, cucumber,
watermelon, muskmelon, lima bean, squash and pumpkin. The seed of very tender
vegetables will rot and die in cold, wet soils.

Protecting Warm Season Vegetables

If a frost is predicted or temperatures in the mid 30 to 40 degree range, it
is wise to cover plants for protection. There are many ways to protect plants
from cold damage.

Frost Caps - If the plants are relatively small and compact, frost caps,
buckets or other containers can be placed directly over the plants to provide
some insulation. The container should be removed the next day before
temperatures get too warm inside the container and cook sensitive plants.

Clear Plastic Wraps - Clear plastic can be wrapped around plants in cages
to protect them from frost damage. The clear plastic works well around early
planted caged tomatoes. The plastic allows light to penetrate during daylight
hours and maintain higher temperatures during cooler days and nights. It is
important to remove plastic on hot days to prevent damage to plants.
Temperatures can climb inside of these plastic tubes under intense sunlight.

Water walls- There are various clear plastic devices that have long tubes
connected in a circle. Once the tubes are filled with water, they will stand up
on their own and provide a great deal of frost protection for early tomatoes.
The water filled tubes heat up during the day and retain heat during cold nights
to protect plants. These thick water walls provide effective insulation to
counter cool temperatures. The water walls need to be staked to prevent them
from blowing over with strong winds. After frost dates have passed, these
devices can be removed.

Row covers are also sold in garden centers and can provide a marginal
level of protection. Avoid placing row covers directly on plants, use hoops or
sticks to hold the row cover slightly above the plant.

Moveable pots – Some gardeners plant tomatoes and peppers in medium to
large pots and bring the plants inside during cold nights. I know of one
gardener who places large pots on a home-made wood platform with rollers on the
bottom and wheels his plants out during the day and back in his garage during
cold nights. He moves his plants to maximize their exposure to sunlight and
therefore maximize yields.

Warming Cold Soils

For those avid gardeners that demand early tomatoes, it is important to
remember that even if we protect the top of the plant from freezing, the soils
need to be warmed up to stimulate adequate plant growth. One way to warm cold
spring soils is to use a dark colored mulch such as black plastic. Lay down
black plastic mulch in the early spring to allow the daytime sun to warm soils.
These warmer soils will jump start root growth on warm season vegetables crops
to insure earlier harvests. For early tomatoes plant short season varieties that
form smaller but mature tomatoes in less time.